1. Field of the Invention
The present invention relates to a voltage measuring apparatus configured to measure voltages of respective battery cells of a battery cell array including a plurality of battery cell groups, each including a predetermined number of battery cells connected in series.
2. Description of the Related Art
As batteries installed in hybrid electric vehicles (HEV) and electric vehicles (EV), highly-efficient lithium ion batteries that are higher in output voltage and energy density than secondary batteries are used in many cases. However, it is difficult to control charging and discharging of the lithium ion batteries, and the lithium ion batteries are at risk for explosion or fire. Therefore, in the case of using the lithium ion batteries as in-vehicle batteries, the voltage management of the batteries is especially important.
FIG. 5 is a circuit diagram showing a schematic configuration of a conventional battery system. As shown in FIG. 5, a conventional battery system includes an in-vehicle lithium ion battery and a voltage measuring apparatus configured to measure the voltage of the lithium ion battery. The in-vehicle lithium ion battery is constituted as a battery cell array 101 including a plurality of battery cell groups Gj connected in series, each of the plurality of battery cell groups Gj including a predetermined number of battery cells Cji (Here, j equals 1 to m, and i equals 1 to n. In an example of FIG. 5, m equals 4) connected in series. With this, since all the battery cells Cji are connected in series, the battery cell array 101 can obtain a high voltage for vehicle drive. To measure the voltages of respective battery cells Cji of the battery cell array 101, a voltage measuring apparatus 103 includes a plurality of measuring units Mj each provided for each of the plurality of battery cell groups Gj. A lower voltage and upper voltage of the battery cells Cji connected to each measuring unit Mj are input to the measuring unit Mj. To be specific, the measuring unit Mj detects the voltages (measured voltages) of the battery cells Cji connected thereto by subtracting the lower voltage of the battery cell Cji from the upper voltage of the connected battery cell Cji. A highest voltage input to the measuring unit Mj (highest voltage of the battery cell group Gj) is set to a power supply voltage VDDj, and a lowest voltage input to the measuring unit Mj (lowest voltage of the battery cell group Gj) is set to a reference voltage VSSj. To each of the measuring units Mj, a drive voltage VIOj for driving the measuring unit Mj is input.
A controller (such as a microcontroller) for measurement control of the plurality of measuring units Mj and measured value management may be provided for each of the measuring units Mj, but this causes an increase in cost. On the other hand, known is a configuration in which a single controller 102 controls all the measuring units Mj in such a manner that the plurality of measuring units Mj are connected to one another so as to be able to transmit signals to one another and receive the signals from one another. The adjacent measuring units Mj are connected to each other by a so-called daisy chain connection, that is, the adjacent measuring units Mj are connected to each other so as to be able to communicate with each other through a communication channel constituted by a communication line pair. The communication line pair includes: an upward communication path SUkj through which current communication from a lower-stage (low voltage-side) measuring unit Mj to an upper-stage (high voltage-side) measuring unit Mk (k=j+1) is performed; a downward communication path SLjk through which current communication from the upper-stage measuring unit Mk to the lower-stage measuring unit Mj is performed; an input communication line SI through which signals are input from one (lowest-stage measuring unit Mm) of the plurality of measuring units Mj to the controller 102; and an output communication line SO through which signals are input from the controller 102 to the measuring unit Mm.
With this configuration, each of the measuring units Mj measures the voltages of the corresponding battery cells Cji and transmits the measurement result to the controller 102 through the downward communication path SLjk. The controller 102 transmits a drive command through the upward communication path SUkj to the measuring unit Mj corresponding to the battery cell Cji to be measured.
Known is a configuration in which in the conventional voltage measuring apparatus as described above, to suitably maintain the insulation property of a loop circuit including the battery cell array 101 when a certain member has contacted the battery cell array 101, an element (diode) configured to prevent a current from flowing from the low potential-side measuring unit Mj to the high potential-side measuring unit Mk is connected between the adjacent measuring units Mj and Mk (see Japanese Laid-Open Patent Application Publication No. 2009-14498, for example).